1. Field of the Invention
The invention relates to an image forming apparatus for developing a latent image based on each color image onto an image holding material and superimposing and transferring the color images onto a recording medium, thereby forming a color image.
2. Related Background Art
As a conventional image forming apparatus for printing and outputting color image data, there is known an apparatus such that, like a laser beam printer (LBP), a latent image of each line is formed onto a photosensitive material by main scanning means such as a rotary polyhedron or the like for scanning laser irradiation light onto the photosensitive material, an image of each color element of the latent image is formed by using a developing agent of the color element of each of magenta (M), cyan (C), yellow (Y), black (BK), and the like, and the image of each color element is overlappingly transferred onto a paper fixed on a transfer drum, thereby forming a color image. An apparatus such that an image of each color element formed on the photosensitive material is color overlapped onto an intermediate transfer material and a color image on the intermediate transfer material is transferred onto a paper in aggregate has also been proposed.
In those apparatuses, the photosensitive material and the transfer drum or intermediate transfer material are driven at a predetermined speed in the direction (sub scanning direction) which perpendicularly crosses the main scanning direction and the color overlapping process is executed onto the paper on the transfer drum or the intermediate transfer material for every color each time the transfer drum or the intermediate transfer material is rotated once. Further, an apparatus such that the image of each recording color element is overlappingly formed on the photosensitive material and is transferred onto the recording paper in aggregate has also been proposed.
In those color image forming apparatuses, since the recording of each color element is independently performed, a resist matching of the image recording positions of the colors is performed to prevent a color deviation of the recording images. In the apparatus using the transfer drum or the intermediate transfer material, the photosensitive material and the transfer drum or the intermediate transfer material are driven at an almost equal speed and the latent image formation of each color is executed at a timing when the area where the image on the transfer drum or the intermediate transfer material is recorded is rotated and conveyed to a predetermined position for the transfer position, thereby performing the resist matching of the image recording positions of the colors. In the apparatus for color overlapping the image onto the photosensitive drum, the operation of the image formation is started when the head of the image forming area on the photosensitive material reaches a predetermined position for the image forming position of each color, thereby performing the resist matching of the image recording positions of the colors.
FIG. 27 is a diagram for explaining a construction of a resist matching control of the conventional image forming apparatus.
In the diagram, reference numeral 1 denotes an image writing timing control circuit for modulating a semiconductor laser 2 in accordance with each of color image signals [yellow (Y), magenta (M), cyan (C), black (BK)] which are transferred from an external device 18 such as image scanner, computer, or the like. The semiconductor laser 2 irradiates a laser beam by the driving of the image writing timing control circuit 1. Reference numeral 3 denotes a rotary polyhedron (polygon mirror) for reflecting the laser beam irradiated from the semiconductor laser 2. Reference numeral 4 denotes an f-.theta. lens for f.theta. correcting the laser beam reflected by the polygon mirror 3 and scanning and exposing onto a photosensitive drum 5, thereby forming an electrostatic latent image onto the photosensitive drum 5. Reference numeral 6 denotes a BD sensor which is disposed near a one-line scanning start position of the laser beam on the photosensitive drum 5 and detects a line scan of the laser beam and generates a main scanning start reference signal BD (BD signal) of each line. The image writing timing control circuit 1 drives the semiconductor laser 2 synchronously with the BD signal outputted from the BD sensor 6 in accordance with each color image signal which is transferred from the external device 18.
Developing devices of yellow (Y), magenta (M), cyan (C), and black (BK) (not shown) are arranged around the photosensitive drum 5 and develop toner images corresponding to the electrostatic latent images of yellow (Y), magenta (M), cyan (C), and black (BK) formed on the photosensitive drum 5.
Reference numeral 7 denotes a transfer drum for transferring toner images developed on the photosensitive drum 5 onto a recording paper 8 wrapped around the transfer drum 7. Reference numeral 9 denotes an optical sensor (sensor) such as a photointerrupter or the like. When a flag 10 fixed to the transfer drum 7 passes through the sensor 9, the light is shielded, so that the optical sensor 9 generates a sub scanning sync signal ITOP (ITOP signal) of each of the colors [yellow (Y), magenta (M), cyan (C), black (BK)]. The photosensitive drum 5 is rotated in the direction shown by an arrow in the diagram by a photosensitive drum driving motor (not shown). Further, the transfer drum 7 is interlocked with the photosensitive drum 5 through a gear and the like and is rotated in the direction of an arrow (sub scanning) at an equal speed synchronously with the photosensitive drum 5.
Reference numeral 19 denotes a delay circuit for transmitting a page sync signal PS obtained by delaying the ITOP signal outputted from the sensor 9 by a predetermined time T to the image writing timing control circuit 1.
Reference numeral 17 denotes a polygon motor for rotating the polygon mirror 3, for example, an octahedron which is axially supported. Reference numeral 16 denotes a phase locked loop (PLL) circuit for controlling the driving of the polygon motor 17 so as to always make the phase of a reference clock CLK that is generated from an oscillator (for example, quartz vibrator) 11 and the phase of an FG pulse (FG signal) that is generated from the polygon motor 17 coincident.
FIG. 28 is a timing chart showing a signal timing in each section shown in FIG. 27.
In the diagram, t1 and t2 denote times from the leading edge of the page sync signal PS obtained by delaying the ITOP signal outputted from the sensor 9 by the predetermined time T by the delay circuit 19 and outputting to an image signal output timing.
The operation of each section will now be described hereinbelow.
The image signal which is sent from the external device 18 such as image scanner, computer, or the like is supplied to the image writing timing control circuit 1. The image writing timing control circuit 1 modulates the semiconductor laser 2 in accordance with the image signal of each of magenta (M), cyan (C), yellow (Y), and black (BK). The laser beam which is irradiated by the semiconductor laser 2 is reflected by the rotary polyhedron mirror (polygon mirror) 3, is f.theta. corrected by the f-.theta. lens 4, scans on the photosensitive drum 5, and forms an electrostatic latent image onto the photosensitive drum 5.
The BD sensor 6 is disposed near the scan start position of one line of the laser beam, detects the line scan of the laser beam, and forms the BD signal of each line (FIG. 28). The BD signal is used to decide an output timing of the image signal of each line in the image writing timing control circuit 1 and is sent as a line sync signal to the external device 18, thereby obtaining the synchronization of the image transmission of each line from the external device 18.
Further, developing devices of magenta (M), cyan (C), yellow (Y), and black (BK) (not shown) are arranged around the photosensitive drum 5. While the photosensitive drum 5 rotates four times, the four developing devices alternately come into contact with the photosensitive drum 5 and develop images by toners corresponding to the electrostatic latent images of M, C, Y, and BK formed on the photosensitive drum 5. The toner images developed by the developing devices are transferred onto the recording paper 8 wrapped on the transfer drum 7.
The sensor 9 to generate an ITOP signal showing the head position of the recording paper 8 on the transfer drum 7 is arranged at the transfer drum 7. When the transfer drum 7 rotates and the flag 10 fixed to the transfer drum 7 passes through the sensor 9, the ITOP signal of each color in FIG. 28 is formed. The photosensitive drum 5 is rotated in the direction shown by an arrow by a photosensitive drum driving motor (not shown). Since the transfer drum 7 is connected to the photosensitive drum 5 through gears, it is rotated in the arrow direction (sub scanning direction) at an equal speed synchronously with the photosensitive drum 5.
The ITOP signal is delayed by a predetermined time by the delay circuit 19 and is supplied as a page sync signal PS to the image writing timing control circuit 1 and external device 18. As for the image signal generated synchronously with the page sync signal PS, a delay amount T of the delay circuit 19 is determined so that it is recorded to the head of the recording paper 8. As shown in FIG. 28, the PS signal is outputted after the elapse of only a predetermined delay time T after the detection of the leading edge of the ITOP signal.
To output an image of each line by the external device 18 or image writing timing control circuit 1 synchronously with the scan of every line by the laser, as shown in FIG. 28, the image signal is irradiated as laser modulation light onto the photosensitive drum 5 synchronously with the BD signal of the line which first arrives after the input of the page sync signal PS.
In the above conventional technique, however, although the BD signal is generated by the rotation of the polygon motor 17, since the ITOP signal and page sync signal PS are generated by the rotations of the photosensitive drum 5 and transfer drum 7, a generation timing of the BD signal and generation timings of the ITOP signal and page sync signal PS are not synchronized.
Therefore, the image signal output timing of each recording color is deviated by up to one line from the leading edge of the page sync signal PS indicative of the paper edge. A color deviation of up to one line occurs in the recording image obtained by overlapping the recording colors, so that there is a problem that an image quality deteriorates.
When the image is recorded, a method of controlling the photosensitive drum driving motor synchronously with the rotation of the polygon motor 17 so as to make the phases of the BD signal and ITOP signal coincide is also considered. However, since it is necessary to control the photosensitive drum driving motor at the absolute position, there are problems such that its control method is complicated, it is difficult to maintain the stability of the apparatus, costs are high, and the like.
Further, a method whereby the ITOP signal is used and, when the photosensitive drum 5 reaches a predetermined position, by resetting the reference clock CLK of the polygon motor 17, the phases of the BD signal and ITOP signal are made coincide has also been proposed.
However, since the rotation of the polygon motor 17 is suddenly changed in accordance with the position of the photosensitive drum 5, it takes time until the rotation of the polygon motor 17 is stabilized. There is, however, a problem such that when the speed of the apparatus rises, the formation of a next image has to be started before the rotation is stabilized, so that an image quality deteriorates.